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Marky

Ground Loops and Cables

26 posts in this topic

My second topic, and again, not sure if it's in the right place.

My MOTU 828mk2 is still keeping me very satisfied. But... the laptop it is connected to is a source of mucho ground loop noise. It's really rather horrid.

So... anyone here care to shed some light (in simple terms for us n00bs) on what is a ground loop, what causes it, and how to get rid of it?

I have a Behringer DI 20 box but not enough cables. Will this do the job, with its ground lift mode? If so, should I be looking for any particular cables to use with it?  It's TRS out from MOTU to TRS in on DI 20 and XLR out from DI20 to XLR in on Subwoofer (which has a built in crossover), if that makes any difference. Are there likely to be any sound issues to be aware of when using a Deadringer DI20?

Do I need to ground lift every source of ground loop noise, or is one in the circuit enough? For example, I run a sep PC for net and mp3 etc. It has a Terratec soundcard, which runs into an input pair on the 828. This net PC also creates a bit of ground loop - though nothing compared to the HP lappie. Is one DI20 on the line between the 828mk2 and the sub enough to eliminate ALL ground loop noise?

OR, alternatively, can I bypass the DI20 altogether, by using some special type of cable (TRS outof 828 to XLR in of Sub). I vaguely recall a former mod on TSWDNSO saying something about balanced cables solving the problem. Is this true?

Sorry for the n00b questions, but I never really learned enough about the various cables available. Any guidance greatly appreciated.

Mark

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are all your leads balanced to start with? Ie: Stereo (2 black stripes on the tip) for the jack leads

xlr-xlr cables are balanced cables - you can get xlr-1/4" balanced and unbalanced, so check how may black lines are on the tip of the xlr-jack cable. 2 is balanced, 1 isn't

balance all the cables first see if that stops the hum.

you could just put the DI in between the lappie and the 828 to balance that signal to see if that reduces it (remember, you need 'balanced' cables from di to s/card)

is the audio from your net pc routed thru the motu as well?

I've not had any probs with ground loops in the studio. Could be all balanced signal path? Clean wiring? Fluke?

Now live on the other hand...

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Ok, thanks CT.

To elaborate, HP Lappie connects to MOTU 828 via firewire. 828 main outs 1/4" to the KRK v8 Sub's XLR in. So no way to put DI Box in there.

The dekstop PC Terratec soundcard output is stereo headphone, I run a jack to RCA cable, with RCA to 1/4" converters on the end. These plug into paired analog ins on the 828.

When the HP Lappie is off, the MOTU acts as a standalone mixer. The ground loop noise is noticeable, but not unbearable. I assume this is the Terratec/PC noise.

However, when the HP Lappie is ON, all hell breaks loose, and the ground loop becomes almost intolerable. And as the lappie is my "studio" machine, I want to eliminate this noise as best I can.

I am at work now - will have to check the cables tonight. Cheers big for the advice on that.

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Question: Is all your equipment running of a single power point in the house (ie. use multi-outlet power boards to connect several pieces of gear to a single mains outlet)?

If not, do so. :)

If pain persists, and I take it the sound dissapears (while the laptop is switched on) when you disconnect the cable running from your PC's soundcard to the 828, then try one of these in-line...

productLarge_93.jpg

Car Stereo Line Isolation Transformer

Under $20! I'm running one between my laptop and my mixer and it's completely elliminated the disk-access and mouse-movement noise I was getting before. Wiped about 2dB off the level, but no obvious signal degradation, though I've not done any critical listening.

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In a hurry, so can only reply briefly for now, but I must bust a few myths.

Balanced cables will make absolutely no difference to ground loops. They will however reduce induced noise picked up along the cable run (which is different noise all together).

A ground loop is the result of your studio having 2 seperate paths to power ground (not signal ground). The resulting hum is due to the 2 power grounds, or phases sitting at slightly different voltages.

Switchmode (Multi input voltage) laptop power supply's with an earth pin almost always cause problems also.

Lets say power ground 1 is 240v between earth and active. Power ground 2 is 243v between earth and active. This means that when you connect 2 audio devices together that are sitting on the different grounds you have an instant 3vAC 50Hz flowing between them due to the 0v signal ground rails being 3v apart. This is why you hear a 50Hz hum.

Secondly, the in line isolators mentioned above, use very cheap wound transformers that do terrible things to your audio waveform like suppress transients and affect frequency response. Avoid them in the studio at all costs.

Make sure everything is running from the same phase in your sub box for starters. Systematically unplug everything one by one to find the culprit. Safely solving ground issues can be a dark art.

Maybe I will have to add an article to the tutorials on this?

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I learnt something today. Great answer ant!

kinda wondering if this should be in another thread, as its not kinda but is about audio interfaces...

went hunting for a general h'ware issues area to dump this and couldn't find one.

i think the info is still relevant if ppl don't care where it goes.

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CT since it's a general purpose topic I'll move it to the Production & Engineering thread. If anyone disagrees then move it back!  ;)

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Thanks gang. I think this is an issue that wil be of use to more users as the community grows.

not looking forward to the great studio rewire of 2007!

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In a hurry, so can only reply briefly for now, but I must bust a few myths.

Balanced cables will make absolutely no difference to ground loops. They will however reduce induced noise picked up along the cable run (which is different noise all together).

A ground loop is the result of your studio having 2 seperate paths to power ground (not signal ground). The resulting hum is due to the 2 power grounds, or phases sitting at slightly different voltages.

Switchmode (Multi input voltage) laptop power supply's with an earth pin almost always cause problems also.

Lets say power ground 1 is 240v between earth and active. Power ground 2 is 243v between earth and active. This means that when you connect 2 audio devices together that are sitting on the different grounds you have an instant 3vAC 50Hz flowing between them due to the 0v signal ground rails being 3v apart. This is why you hear a 50Hz hum.

Secondly, the in line isolators mentioned above, use very cheap wound transformers that do terrible things to your audio waveform like suppress transients and affect frequency response. Avoid them in the studio at all costs.

Make sure everything is running from the same phase in your sub box for starters. Systematically unplug everything one by one to find the culprit. Safely solving ground issues can be a dark art.

Maybe I will have to add an article to the tutorials on this?

Great answer re ground/earth loops. ;)

re Laptop power supplies.

I've had two Mac laptops, and both laptops emit buzzing and all sorts of rubbish related to the movement of the mouse/drive from the in-built soundcard when the laptop's plugged into the mains and a USB and/or FireWire cable is connected to a (powered) MIDI Interface and/or (powered) Audio Interface. N.B. All equipment runs from a single mains outlet.

What do you suppose is the issue/solution?

I took a punt on that "Car Stereo Line Isolation Transformer" and was pleasantly suprised how clean it was, even running one channel connected, one channel bypassed, and there was no obvious degradation other than around -2dB attentuation.

Having said that, I'm not relying on it for anything critical, just system sounds and iTunes (when not streaming it to the other side of the house over WiFi). When I'm up for it, I'll run some more tests.

I know such a passive device is just asking for the audio quality to suffer, however, like I said, I was pleasantly suprised how clean the sound was coming from it. The top end and distortion to transients being my focus.

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The line isolation transformer is a big inductor. Think about how inductors work, and you'll have your answer on why it will degrade the signal... especially HF :angry:

WRT laptop Switchmode PSU's, thie biggest problem is cheap design. The issues seem to be generated from the fact some manufacturers insist on running the ground plane common between the 110/240V input side of the supply and the DC output. It's really bad design, they're just being lazy and making no effort to electrically isolate the 2 sides of the supply. In some cases, some tool gets it in their head that having the ground go through to the PC also provides some sort of safety protection in the event the PSU fails and applies AC volatge to the metal computer parts. Again, bad design - you can safely engineer that shite out.

The solution?

Most of all DO NOT remove the ground pin from your laptop PSU. If you think this is the solution, take it to an electrician or engineer to have the pin removed and the PSU retested to ensure it's safe. The real solution is to kick some mail at Apple for their continued incompitence... and look for an after market PSU that's only 2 pin - meaning they've built their own DC ground reference. I woul;dn't be surprised if Apple sell one as an aftermarket 'option.

I disagree with Ant that balanced cables will make no difference to loop issues. If the balanced cable has ground connected at BOTH ends, then there's a potential source for a ground loop. The 'trick' with removing hum using balanced cables is to remove the ground at one end of the cable... removing the loop path. On the other hand, when you're talking about a ground reference loop (where, as Ant described, one voltage has high potential difference than the other), you may find balanced cables DO help as they can, depending on the circuit design of your audio sources, create that direct reference link between the two, shifting them to the same potential.

That's a way to cure a ground reference loop - you connect the earth of the two humming devices together to bring them to the same potential difference. You've got to remember that in most cases, it's not the AC input voltage difference directly causing the hum - it's the DC output inside your mixers that uses the same earth as the input AC voltage. That's why it's a ground reference loop - the two devices are seeing 2 different potentials at ground. In Ant's example, one ground appears to be shifted 3Volts AC to the other... and you get the noise.

So, summarising that BS:

* Try balanced cables first - you may find they create a reference point between all your source bringing them to the same ground potential and curing the loop.

* If you're already using balanced cables, you may be creating the loop. Try lifting the sheild from one end of each connector, removing the earth reference. This is more likely the solution if your DC supplies are creating an adde3d reference issue in a domestic situation.

* Isolation should only be a last resort.

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The line isolation transformer is a big inductor. Think about how inductors work, and you'll have your answer on why it will degrade the signal... especially HF :angry:

Yep, totally with you on that one. And frustrated that my aging ears (rocks in my head?) have failed to hear any obvious losses. Mind you, as I've said, I'm not using it for anything critical, and I spent only a few minutes with mp3s on iTunes and didn't spot any issues.

And I totally agree that Apple need a serious boot up 'em re this soundcard noise issue. I thought my iBook was bad.. the Mac Book Pro was worse. I'm sure they'd be quick to blame my peripherals though. Could my AMT8 MIDI interface/power supply be bad? It runs off a DC plug-pack???

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Yep, totally with you on that one. And frustrated that my aging ears (rocks in my head?) have failed to hear any obvious losses. Mind you, as I've said, I'm not using it for anything critical, and I spent only a few minutes with mp3s on iTunes and didn't spot any issues.

Does that say more about your ears, or the Kwality of MP3's :angry:

Could my AMT8 MIDI interface/power supply be bad? It runs off a DC plug-pack???

Yes. Have you tried losening a screw on the case for the ATM and running a cable to an earth point on the mac book (like a screw or the PSU -ve)? Does it respond the same when you're on battery or just with the PSU plugged in?

For the record - never had any problems like this running a Dell on the 90W 2 pin PSU ;D

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And I totally agree that Apple need a serious boot up 'em re this soundcard noise issue. I thought my iBook was bad.. the Mac Book Pro was worse. I'm sure they'd be quick to blame my peripherals though. Could my AMT8 MIDI interface/power supply be bad? It runs off a DC plug-pack???

that's a bugger - the iMac has such great sound from the headphone jack i assumed it was the same across the board. for my last gig i couldn't be bothered taking my firewire sound card so i just ran a massive RCA to minijack lead into the PA. sounded ace (as i prayed to jeebers it would) but different kettle of fish i'm sure.

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I would like to clear up some mis-information that's been bugging me before I go.

A correctly made balanced cable will not ever stop a ground loop. The earth should be connected at both ends, with the shielding connected only at the input side of the cable.

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I would like to clear up some mis-information that's been bugging me before I go.

Calm down dude. Seriously. If everyone who got cut by Jester once quit the internets then it'd be tumbleweed city.

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Try using the other plug for the PSU phil, it only has 2 pins.

Cheers. I only just realised the extension cord (3 pins) versus the adaptor plug (2 pins).

Shoudn't make a difference though as even the 3-pin extension only connects with two contacts at the transformer/powersupply...

...which means there's no "2-pin" solution as it's already running 2-pins, no earth. ;)

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Cheers. I only just realised the extension cord (3 pins) versus the adaptor plug (2 pins).

Shoudn't make a difference though as even the 3-pin extension only connects with two contacts at the transformer/powersupply...

...which means there's no "2-pin" solution as it's already running 2-pins, no earth. ;)

Its just funny, as i dont have any issues...

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Its just funny, as i dont have any issues...

And nor it would it seem others, as it's not something the world are complaining about.

If you have  a moment, could you please run through what you have hooked up to what with your set-up?

Eg. are you running *both* an audio interface and the soundcard into a mixer/amp/speakers?

I've racked my brains to think what am I doing that might be so unusual. If I plug headphones directly into the soundcard, there's no issues, only when the soundcard is connected up to my mixer (while mains-powered USB/FireWire gear is also plugged in).

Cheers.

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hey spec,

is the noise just from moving your mouse? Touchpad as well? MIDI on AMT8?

Have you tried a powered USB hub?

Maybe AMT8 is drawing some power from the internal USB even when plugged into mains.

Thought beefing up the USB power might help... Has worked for me on my old ASUS A2D when I have more than one thing plugged into USB...

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^^^ Yep, it's *that* sort of noise.

And my cheapo Belkin USB is powered too.

Cheers for the suggestions though. If I remember, the issue was also there with just the FireWire cable hooked up to a powered-down Digi002R, that is, with no USB connected.

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Cheers. I only just realised the extension cord (3 pins) versus the adaptor plug (2 pins).

Shoudn't make a difference though as even the 3-pin extension only connects with two contacts at the transformer/powersupply...

...which means there's no "2-pin" solution as it's already running 2-pins, no earth. ;)

Phil, after examining both plug connections, the 2 pin plug is a 2 pin solution as the 3 pin plug has an earth connector. The pin that guides the connector onto the PSU is the earth point for the PSU.

Have a look at the connectors and you will see what i am talking about. So i would use the 2 pin and go from there.

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